fix: OnNetworkDespawn is not called on server side when a client disconnects [MTT-5120] (#2323)

* fix
This prevents the issue (when just destroying the GameObject) where any NetworkBehaviour component(s) destroyed before the NetworkObject would not have OnNetworkDespawn invoked.

* test
Adding the test to validate this fix.

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -15,6 +15,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - The default listen address of `UnityTransport` is now 0.0.0.0. (#2307)
 
 ### Fixed
+- Fixed server side issue where, depending upon component ordering, some NetworkBehaviour components might not have their OnNetworkDespawn method invoked if the client side disconnected. (#2323)
 
 - Fixed an issue in `UnityTransport` where an exception would be thrown if starting a Relay host/server on WebGL. This exception should only be thrown if using direct connections (where WebGL can't act as a host/server). (#2321)
 

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -2082,7 +2082,12 @@ namespace Unity.Netcode
                             }
                             else
                             {
-                                Destroy(playerObject.gameObject);
+                                // Call despawn to assure NetworkBehaviour.OnNetworkDespawn is invoked
+                                // on the server-side (when the client side disconnected).
+                                // This prevents the issue (when just destroying the GameObject) where
+                                // any NetworkBehaviour component(s) destroyed before the NetworkObject
+                                // would not have OnNetworkDespawn invoked.
+                                SpawnManager.DespawnObject(playerObject, true);
                             }
                         }
                         else

com.unity.netcode.gameobjects/TestHelpers/Runtime/NetcodeIntegrationTest.cs

@@ -241,11 +241,20 @@ namespace Unity.Netcode.TestHelpers.Runtime
         {
         }
 
+        /// <summary>
+        /// Invoked immediately after the player prefab GameObject is created
+        /// prior to adding a NetworkObject component
+        /// </summary>
+        protected virtual void OnPlayerPrefabGameObjectCreated()
+        {
+        }
+
         private void CreatePlayerPrefab()
         {
             VerboseDebug($"Entering {nameof(CreatePlayerPrefab)}");
             // Create playerPrefab
             m_PlayerPrefab = new GameObject("Player");
+            OnPlayerPrefabGameObjectCreated();
             NetworkObject networkObject = m_PlayerPrefab.AddComponent<NetworkObject>();
 
             // Make it a prefab

com.unity.netcode.gameobjects/Tests/Runtime/NetworkBehaviourGenericTests.cs

@@ -1,5 +1,6 @@
 using System.Collections;
 using UnityEngine;
+using NUnit.Framework;
 using UnityEngine.TestTools;
 using Unity.Netcode.TestHelpers.Runtime;
 using Unity.Netcode.Components;
@@ -147,5 +148,35 @@ namespace Unity.Netcode.RuntimeTests
             // (validating the fix)
             Object.Destroy(parentObject);
         }
+
+        protected override void OnPlayerPrefabGameObjectCreated()
+        {
+            // Adds the SimpleNetworkBehaviour before the NetworkObject
+            // for OnNetworkDespawnInvokedWhenClientDisconnects testing
+            m_PlayerPrefab.AddComponent<SimpleNetworkBehaviour>();
+        }
+
+        /// <summary>
+        /// This validates that upon a client disconnecting, the server-side
+        /// client's player clone will invoke NetworkBehaviour.OnNetworkDespawn
+        /// when the component precedes the NetworkObject component.(PR-2323)
+        /// </summary>
+        [UnityTest]
+        public IEnumerator OnNetworkDespawnInvokedWhenClientDisconnects()
+        {
+            m_AllowServerToStart = true;
+
+            // Now just start the Host
+            yield return StartServerAndClients();
+
+            // Now create and connect a new client
+            yield return CreateAndStartNewClient();
+
+            var serverSidePlayer = m_PlayerNetworkObjects[NetworkManager.ServerClientId][m_ClientNetworkManagers[0].LocalClientId].GetComponent<SimpleNetworkBehaviour>();
+
+            yield return StopOneClient(m_ClientNetworkManagers[0]);
+
+            Assert.True(serverSidePlayer.OnNetworkDespawnCalled, $"Server-side player clone did not invoke OnNetworkDespawn!");
+        }
     }
 }